1. Field of the Invention
The present invention relates to a non-flammable liquid rubber composition having high electric insulating property, high arc resistance, high tracking resistance.
2. Description of the Prior Art
In a case of arcing in electric instruments such as a mold transformer and a transformer, organic materials used as insulators are decomposed to form a carbon powder which forms a low electric resistant path to cause fault of the electric instrument in a short time.
The electric instruments are usually used for a long time in various environments at high temperature in high humidity with much dust, and accordingly, sometimes, they may cause tracking breakdown. Sometimes, a firing fault is found by arcing caused by a sudden short-circuit. Thus, the mold composition used as the insulator should have high electric insulating property, high arc resistance, high tracking resistance and high flame retardancy so as to prevent such fault.
In order to satisfy such required characteristics, it has been well-known to use composition incorporating a conventional molding material such as elastomers such as ethylenepropylene terpolymer (EPT) and butyl rubber; thermosettable resins such as epoxy resin and unsaturated polyester resin and a flame retardant filler such as alumina hydrate (Al.sub.2 O.sub.3.3H.sub.2 O). It has been also known to incorporate a halogen type flame retardant. This method, however causes disadvantages that a desired effect is not expected unless a large amount of the alumina hydrate or the other flame retarder is incorporated and the electric characteristics may be remarkably deteriorated.
The epoxy resin and the unsaturated polyester resin are respectively excellent molding materials which are moldable in a liquid form to be excellent processability, however, their cured products have relatively low arc resistance and low tracking resistance and have low hydrolysis resistance and accordingly, the products are not suitable to be use in outdoor or in an environment at high temperature in high humidity.
On the other hand, EPT rubber and butyl rubber as the elastomer are desired molding materials in view of high insulating property and stability in various environmental conditions. EPT rubber and butyl rubber, however, have large molecular weights and low fluidity and accordingly, they should be molded at high temperature under high pressure whereby the electric instruments should be prepared by a compressive molding method, an extrusion molding method, a transfer molding method, etc. When the electric instruments are prepared by said molding methods, there is a possibility to cause deformation or deviation of the packed part under the condition of high temperature and high pressure or to cause insufficient insulation because the rubber is not completely filled in the space for insulation among coil layers and between a primary coil and a secondary coil as the fluidity of the rubber is not sufficient. In order to prevent such insufficient insulation, it has been necessary to reinforce the packed part against the pressure or to treat it for sufficient insulation before the molding, in the conventional one. Moreover, expensive apparatus and mold are required for the molding at high temperature under high pressure and a large energy consumption is required. The conventional method has not been a desired process for production.
In usual, it has been proposed to incorporate antimony oxide, an organic halogen-containing compound or a phosphoric acid ester at a ratio of several tens % or to blend a polyvinyl chloride or to copolymerize vinyl chloride for the purpose of flame retardant treatment of a polyolefin. Such methods have disadvantages that antimony oxide and phosphoric acid esters are toxic or a halogen-containing gas is formed or electrical or mechanical characteristics are deteriorated depending upon the kind and amount of the flame retarder. As the other flame retardant method, it has been proposed to incorporate an inorganic compound having crystal water. In such case, a large amount of the inorganic compound should be incorporated for imparting sufficient flame retardancy whereby the mechanical characteristic of the resin composition is remarkably inferior. The above-mentioned flame retarding manners have been considered to be useful for the flame retardancy of the olefin type resin depending upon the usages, however, there are many problems for overcoming said disadvantages.